THE NERVOUS SYSTEM 



415 



of the development of the five areas of the brain, as shown in 

 the different vertebrate Classes, may be studied by the help of 

 the accompanying diagrams [Plates IV and V], which rep- 

 resent the adult brains of a dog-fish, a teleost, an amphibian, a 

 reptile, a bird and a mammal, sectioned sagittally in the median 

 plane and viewed from the inner aspect. The comprehension 



FIG. 116. Diagram of the cerebral flexures. 



Angle between axes ab and cd apical flexure. Angle between axes cd and 

 ef = flexure of the pons. Angle between ef and gh = cervical flexure. 



of these will be facilitated by comparing them with Fig. 117, 

 which shows the dorsal aspect of the same series. 



The telencephalon in the dog-fish is inconspicuous in size but 

 of a considerable thickness, which is approximately uniform 

 save at the posterior part of the roof, where a considerable 

 area retains its membranous character and invaginates to form 



